Most orthodontist appointments are around 20 minutes tops - the initial fitting one is usually a half day and it's quite common for the afternoon or following morning to be taken off due to discomfort. Other than that, the emergency 'a wire's snapped' appointments have been done by many so quickly that the kids have left, been driven there, had it done and been driven back before the end of the lesson.
We know it happens, we know it's the realistic time/age for kids to need authorised absences for orthodontist appointments. It's not the end of the world, especially now most schools will put the work for the week onto the online learning by Monday morning. A ten thirty appointment will usually see them back before lunch.
Obviously, if somebody starts taking entire days off for a wire fix, it's not a good thing, but in all, missing an hour every 2 months isn't a disaster - and it's better than the kids last year who had to go months without access to orthodontics, so we had multiple kids out every single day when they'd not even been in school for a week.
And leaving the morning/lunchtime/evenings to adults means that staff can get dental appointments without affecting cover as much.
Tl;dr it happens. No point getting het up about it.